The present application claims priority under 35 U.S.C. xc2xa7119 to Japanese Patent Application No. 11-270201, filed Sep. 24, 1999, entitled xe2x80x9cArrayed Waveguide Gratingxe2x80x9d, Japanese Patent Application No. 2000-021533, filed Jan. 31, 2000, entitled xe2x80x9cArrayed Waveguide Gratingxe2x80x9d, and Japanese Patent Application No. 2000-219205, filed Jul. 19, 2000, entitled xe2x80x9cArrayed Waveguide Gratingxe2x80x9d. The Contents of these applications are incorporated herein by reference in their entirety.
1. Field of the Invention
The present invention relates to an arrayed waveguide grating (AWG) which is used as, for example, an optical multiplexer or demultiplexer. Further, the present invention relates to a method for compensating an optical transmitting center wavelength of light which travels through an arrayed waveguide grating.
2. Discussion of the Background
In the field of optical communications, active researches and developments of the WDM (Wavelength Division Multiplexing) optical communications have been made over the recent years in order to dramatically increase transmission capacity. According to the WDM optical communications, for example, plural fluxes of light having different wavelengths are transmitted in multiplexing. The WDM optical communications system includes an optical transmitting device that transmits only the beams of light having predetermined wavelengths in order to extract the light beam of each wavelength from the multiplexed beams at a light receiving side.
FIG. 11 shows an arrayed waveguide grating (AWG) of a planar lightwave circuit (PLC) by way of one example of the optical transmitting device. The arrayed waveguide grating has a waveguide pattern as illustrated in FIG. 11. The waveguides include cores and claddings composed of silica-based glass or the like. The waveguides are provided on a substrate 1 which is made of silicon or the like.
In the waveguide pattern of the arrayed waveguide grating, a first slab waveguide 3 is connected to an exit side of one or more optical input waveguides 2 provided in a side-by-side relation. A plurality of arrayed waveguides 4 provided side by side are connected to an exit side of the first slab waveguide 3. A second slab waveguide 5 is connected to an exit side of the arrayed waveguides 4. A plurality of optical output waveguides 6 provided side by side are connected to an exit side of the arrayed waveguides 4.
The arrayed waveguides 4 serve to transmit the light traveling through the first slab waveguide 3. The arrayed waveguides 4 are formed to have different lengths. The lengths of the arrayed waveguides 4 adjacent to each other are different by (xcex94L). Note that the optical input waveguides 2 and the optical output waveguides 6 are provided corresponding to the number of signal lights which have wavelength different from each other and which are demultiplexed or multiplexed by, for example, the arrayed waveguide grating. Normally, the arrayed waveguides 4 include a lot of waveguides, for example, 100 waveguides. Referring to FIG. 11, however, simply countable numbers of the optical input waveguides 2, the arrayed waveguides 4 and the optical output waveguides 6 are shown therein for simplicity of illustration.
Since optical fibers (not shown) of, for example, a transmission side are connected to the optical input waveguides 2, the WDM light is introduced. The light entering the first slab waveguide via the optical input waveguides 2 expands due to a diffraction effect thereof and enters the respective arrayed waveguides 4, thus traveling through the arrayed waveguides 4.
The light traveling through the arrayed waveguides 4 arrives at the second slab waveguide 5. Then, these fluxes of light are converged on and outputted to the optical output waveguides 6. However, all the arrayed waveguide 4 have their lengths different from each other. Accordingly, there occur phase shifts between the individual beams of light after traveling through the arrayed waveguides 4. A phase front of the converged flux of light inclines corresponding to a quantity of this phase shift, and a position of the convergence is determined based on an angle of this inclination.
Therefore, the converging positions of the beams of light having different wavelengths become different from each other. The optical output waveguides 6 are provided in those different converging positions. Accordingly, the demultiplexed beams of light having different wavelengths may be outputted from the optical output waveguides 6 provided in the positions different according to the respective wavelengths.
Namely, the arrayed waveguide grating incorporates an optical demultiplexing function of demultiplexing the beams of light having one or more wavelengths from the multiplexed beams of light inputted from the optical input waveguide 2 and having the plurality of wavelengths different from each other, and outputting the thus demultiplexed beams of light from each of the optical output waveguides 6. A center wavelength of the demultiplexed beams of light is proportional to an effective refractive index (nc) of the optical waveguide 4 as well as to the difference (xcex94L) in length between the arrayed waveguides 4.
The arrayed waveguide grating exhibits the characteristics described above and is therefore used as a WDM demultiplexer for a WDM transmission. For example, as shown in FIG. 11, when WDM light beams having wavelengths (xcex1, xcex2, xcex3, . . . , xcexn) (n is an integer 2 or larger) are inputted from one single line of optical input waveguide 2, the light beams having these wavelengths are expanded through the first slab waveguide 3 and arrive at the arrayed waveguides 4. The light beams travel via the second slab waveguide 5, as described above, converge on the different positions according to the wavelengths and enter the different optical output waveguides 6. The light beams then travel through the corresponding optical output waveguides 6 and are outputted from the exit ends of these optical output waveguides 6.
Then, the optical fibers (not shown) for outputting the light are connected to the exit ends of the optical output waveguides 6. Therefore, the light beams having the above wavelengths are taken out via these optical fibers. Note that when connecting the optical fibers to the optical output waveguides 6 and to the optical input waveguides 2, for instance, an optical fiber array in which the optical fibers are fixedly disposed in a one-dimensional array is prepared and fixed to connection end surface sides of the optical output waveguides 6 and of the optical input waveguides 2. Thus, the optical fibers are connected to the optical output waveguides 6 and to the optical input waveguides 2.
In this arrayed waveguide grating, the light beams outputted from the optical output waveguides 6 exhibit an optical transmitting characteristic (a wavelength characteristic of an intensity of the transmitting light of the arrayed waveguide grating) as shown in FIG. 12. Referring to FIG. 12, an optical transmission becomes smaller as the wavelength shifts from the corresponding optical transmitting center wavelength (e.g., xcex1, xcex2, xcex3, . . . , xcexn). It should be noted that the optical transmitting characteristic does not necessarily have one maximal value and might have two or more maximal values in some cases.
Further, the arrayed waveguide grating utilizes the principle of the light reciprocity (reversibility), and therefore has a function of an optical demultiplexer and a function of an optical multiplexer as well. That is, in a direction reverse to the direction in FIG. 11, the light beams having a plurality of diferrent wavelengths enter the optical output waveguides 6 corresponding to the respective wavelengths, then travel through the transmission path in the reverse direction. These light beams are multiplexed in the arrayed waveguides 4 and exit through one single optical input waveguide 2.
In the thus constructed AWG, as explained above, the wavelength resolution of the grating is proportional to the difference (xcex94L) in length between the arrayed waveguides 4 constituting the grating. Hence, the WDM light having wavelengths with small differences may be multiplexed and demultiplexed by increasing the difference (xcex94L), which could not be realized in the conventional AWG. This design makes it feasible to perform the multiplexing/demultiplexing function for plural beams of signal light which is required for actualizing high-density WDM optical communications, i.e., the function of demultiplexing or multiplexing a plurality of optical signals whose wavelength differences are at most 1 nm.
When manufacturing the AWG described above, for instance, an under cladding layer and a core layer are formed on a silicon substrate by a flame hydrolysis deposition method. Then, sintering and vitrifying thereof are effected. Thereafter, photolithography is carried out through a photomask depicted with a waveguide pattern of the AWG, and the AWG pattern is transferred onto the core layer by a reactive ion etching method. Thereafter, an over cladding layer is formed by using the flame hydrolysis deposition method once again, and sintered and vitrified, thereby manufacturing the AWG.
The AWG is mainly made of a silica-based glass material which has a temperature dependency. Accordingly, the optical transmitting center wavelength of the AWG shifts corresponding to a change in the AWG temperature. This temperature dependency is expressed by the following formula 1:                                           ⅆ            λ                                ⅆ            T                          =                                            λ                              n                c                                      ⁢                                          ⅆ                                  n                  c                                                            ⅆ                T                                              +                      λ            ⁢                          xe2x80x83                        ⁢                          α              s                                                          [                  Formula          ⁢                      xe2x80x83                    ⁢          1                ]            
where (xcex) is the optical transmitting center wavelength of the light beam outputted from the single optical output waveguide 6, (nc) is an effective refractive index of the core that forms the arrayed waveguide 4, (xcex1s) is a thermal expansion coefficient of the substrate (e.g., silicon substrate) 1, and (T) is a temperature change of the AWG.
Herein, in the typical AWG, the temperature dependency of the optical transmitting center wavelength is obtained from the formula (1). The parameters in this AWG are given such as dnc/dT=1xc3x9710xe2x88x925 (xc2x0C.xe2x88x921), xcex1s=3.0xc3x9710xe2x88x926 (xc2x0C.xe2x88x921), nc=1.451 (a value when the wavelength is 1.55 xcexcm). These values are used in the formula (1).
Further, although the wavelength (xcex) differs corresponding to each of the optical output waveguides 6, the temperature dependency of each wavelength (xcex) is the same. Then, the AWG prevailing at the present is used in many cases for demultiplexing and multiplexing the WDM light in a wavelength band in the vicinity of a wavelength of 1550 nm, and hence xcex=1550 nm is herein used in the formula (1). The temperature dependency of the optical transmitting center wavelength in the typical AWG is shown by the formula (2):                                           ⅆ            λ                                ⅆ            T                          =                  0.015          ⁢                      xe2x80x83                    ⁢                      (                          nm              ⁢                              /                            ⁢              xc2x0              ⁢                              xe2x80x83                            ⁢                              C                .                                      )                                              [                  Formula          ⁢                      xe2x80x83                    ⁢          2                ]            
Note that a unit of dxcex/dT is nm/xc2x0C. Supposing that the temperature of the AWG changes, for example, by 20xc2x0 C., the optical transmitting center wavelength of the light beam outputted from the single optical output waveguide 6 shifts to 0.30 (nm). If the temperature changes, for example, 70xc2x0 C. or higher, the shift amount of the optical transmitting center wavelength becomes 1 (nm) or greater.
The AWG may demultiplex or multiplex the wavelength at a wavelength differences of as small as 1 (nm) or under and thus is applied to the WDM optical communications. It is therefore a problem arises in that the optical transmitting center wavelength, as explained above, shifts by the amount described above due to the change in the environmental temperature.
Under such circumstances, there has hitherto been proposed an arrayed waveguide grating including a temperature control mechanism for keeping a temperature of the AWG constant so that the optical transmitting center wavelength does not shift due to the temperature change. This temperature control mechanism includes, for example, a Peltier device or a heater. The temperature control mechanism control the temperature of the AWG to maintain at a temperature higher than a room temperature.
In the AWG shown in FIG. 11, a Peltier device 30 is provided on the side of a substrate 1 of the AWG, and controls the temperature of the AWG to be constant on the basis of a temperature detected by thermistor 31.
Further, a temperature control mechanism including a heater as a substitute for the Peltier device controls the temperature of the AWG to be a constant high temperature.
When the temperature of the AWG is thus kept constant, neither an expansion/contraction of the substrate 1 nor a change in effective refractive index of the core occurs. It is therefore possible to compensate the temperature dependency of the optical transmitting center wavelength.
The AWG and the optical fiber array are connected generally by use of a bonding agent. If the temperature of the AWG is controlled at room temperature or higher by using the Peltier device or the heater, the bonding agent applied to the connection surface between the AWG and the optical fiber array, for example, expands or softens due to the temperature. Accordingly, if constructed to keep the temperature of the AWG constant by use of the Peltier device, etc., a problem arises in that there might increase a loss of connection between the optical fibers and the optical input waveguides 2 and optical output waveguides 6 of the AWG because of the expanded or softened bonding agent, and that a reliability of the connection between the AWG and the optical fibers might decline.
According to Japanese Patent Application Laid-open No. Hei 11-218639, entitled xe2x80x9cPhasing Array Device, i.e., Phaser and Method of Manufacturing the Samexe2x80x9d (Claim for Right of Priority Number: 97 13440, Priority Date: Oct. 27, 1997, Country of Claim for Right of Priority: France (FR)), a phasing array device (which is the same as the AWG) includes a first piece and a second piece. The contents of this application are incorporated herein by reference in their entirety. The first piece includes a part of a first slab waveguide and an input waveguides. The second piece includes the other part of the first slab waveguide and other components of the device. The first piece and the second piece are assembled into the complete phasing array device so that a wavelength of the device can be adjusted.
The wavelength of the phasing array device disclosed in Japanese Patent Application Laid-open No. Hei 11-218639 is adjusted with respect to an initial shift of the optical transmitting center wavelength due to a scatter in manufacturing the phasing array device. After the adjustments of the plurality of pieces have been finished, the plurality of pieces are fixedly bonded to the support member with a bonding agent, etc.
According to an aspect of the invention, an arrayed waveguide grating includes at least one first optical waveguide, a first slab waveguide, a plurality of arrayed waveguides, a second slab waveguide, and a plurality of second optical waveguides. The plurality of arrayed waveguides are connected to the at least one first optical waveguide via the first slab waveguide. Each of the plurality of arrayed waveguides has a different length. The plurality of second optical waveguides are connected to the plurality of arrayed waveguides via the second slab waveguide. At least one of the first and second slab waveguides is partitioned to first and second segments at a partition surface intersecting a path of light which travels through the arrayed waveguide grating. At least one of the first and second segments is configured to be slid along the partition surface to compensate an optical transmitting center wavelength of the light according to a temperature of the arrayed waveguide grating.
According to another aspect of the invention, a method for compensating an optical transmitting center wavelength of light which travels through an arrayed waveguide grating includes partitioning at least one of first and second slab waveguides to first and second segments at a partition surface intersecting a path of the light, and sliding at least one of the first and second segments along the partition surface to compensate an optical transmitting center wavelength of the light according to a temperature of the arrayed waveguide grating.
According to yet another aspect of the invention, a wavelength division multiplexing optical communication system includes an arrayed waveguide grating through which light travels. The arrayed waveguide grating includes at least one first optical waveguide, a first slab waveguide, a plurality of arrayed waveguides, a second slab waveguide, and a plurality of second optical waveguides. The plurality of arrayed waveguides are connected to the at least one first optical waveguide via the first slab waveguide. Each of the plurality of arrayed waveguides has a different length. The plurality of second optical waveguides are connected to the plurality of arrayed waveguides via the second slab waveguide. At least one of the first and second slab waveguides is partitioned to first and second segments at a partition surface intersecting a path of light which travels through the arrayed waveguide grating. At least one of the first and second segments is configured to be slid along the partition surface to compensate an optical transmitting center wavelength of the light according to a temperature of the arrayed waveguide grating.
According to yet another aspect of the invention, an arrayed waveguide grating includes a first slab waveguide, a plurality of arrayed waveguides, and a second slab waveguide. The plurality of arrayed waveguides connects the first slab waveguide and the second slab waveguide. Each of the plurality of arrayed waveguides has a different length. At least one of the first and second slab waveguides is partitioned to first and second segments at a partition surface intersecting a path of light which travels through the arrayed waveguide grating. At least one of the first and second segments is configured to be slid along the partition surface to compensate an optical transmitting center wavelength of the light according to a temperature of the arrayed waveguide grating.